A nuclear power plant utilizes the fission energy produced in nuclear fuel. Tens of-hundreds of fuel bodies composed of nuclear fuel materials are loaded in a zirconium-based tube, which is then seal-welded to fabricate a fuel rod. Tens of-hundreds of fuel rods are assembled to make a bundle and these bundles are loaded in water reactors. Heat generated in the fuel body is transferred through the zirconium-based tube to coolant flowing around the fuel rods.
For a nuclear fuel, cylindrical or spherical shape fuel body fabricated by sintering and processing the individual oxide or the mixture of oxides of uranium(U), plutonium(Pu), thorium(Th), etc are used. Uranium dioxide(UO2) is generally used to fabricate a fuel body and one or more materials selected from a group consisting of oxides of Pu, Th, Gd, etc can be added to UO2. Particularly, (U, Pu)O2, (U, Th)O2, (U, Gd)O2, (U, Pu, Gd)O2 or (U, Th, Pu)O2 is preferably used.
The most preferably used nuclear fuel is the uranium oxide sintered body, which is prepared by the steps of mixing lubricant with uranium oxide powder, preparing slug by preliminary processing with 1 ton/cm2 pressure, preparing granules by crushing the slug, pressing the granules into green pellet having about 50% TD (theoretical density), and sintering the green body at 1600-1800° C. for 2-4 hours in hydrogen-containing gas. The produced uranium oxide sintered body has a cylindrical shape and 95% density based on theoretical density. The microstructure of the sintered body is represented in FIG. 1. As shown in FIG. 1, the structure of the sintered body is constructed by polygonal grains having 3˜20 μm in diameter.
In like manner, (U,Pu)O2 or (U,Th)O2 sintered body is prepared by mixing plutonium oxide powder or thorium oxide powder with uranium oxide powder and then by following the above steps as used for preparing the uranium oxide sintered body. (U,Gd)O2 sintered body is also prepared by mixing gadolinium oxide powder with uranium oxide powder and then by following the steps as used for preparing the uranium oxide sintered body. In order to grow grains of nuclear fuel sintered body, one or more oxides selected from a group consisting of Nb, Ti, Si, Mg and Al oxides are added to uranium oxide.
UO2 has been generally used as a nuclear fuel material since it has a high melting point and reacts negligibly with coolant(water). But, UO2 has comparatively low thermal conductivity (2˜5 W/m K). If a nuclear fuel material has low thermal conductivity, heat generated by nuclear fission cannot be transferred to coolant right away, resulting in an increase in the temperature of fuel body. The temperature of fuel body is highest at the center and lowest at the surface. The difference in temperature between the surface and the center of the sintered body is inversely proportional to thermal conductivity. Therefore, the center temperature of fuel body increases as thermal conductivity decreases. The center temperature of fuel body in normally burning nuclear fuel rods ranges from 1000° C. to 1500° C., but if it is not in a normal operation, the temperature can go over 2800° C., which is the melting point of UO2.
Since the nuclear fuel sintered body keeps high temperature, all the temperature-depending reactions are accelerated, resulting in the lowering of material performance. The higher the burning temperature is, the lower the performance becomes. In addition, a high temperature in sintered body lowers the safety margin in anticipated nuclear power plant accidents. In case of rapid elevation of the generating power of fuel body, the center temperature might increase up to over the melting point of UO2. In order to prevent this problem, the generating power ought to be limited, with suffering an economic loss.
The thermal conductivities of (U,Pu) O2, (U, Gd) O2 or (U,Th)O2 are as good as or worse than that of UO2. Especially, the thermal conductivity of (U,Gd)O2 is much worse than that of UO2, suggesting that the temperature of (U,Gd)O2 sintered body is much higher than that of UO2 sintered body under the condition of having same generating power. To solve the problem, the generating power of (U,Gd)O2 sintered body ought to be more limited than that of UO2 fuel body.
Thus, the present invention prepared a novel nuclear fuel sintered body in which tungsten network having a high melting point and excellent thermal conductivity is continuously formed in order to make up for such disadvantage that the nuclear fuel sintered body has a low thermal conductivity.